In connection with measuring physical parameters such as pressure and temperature, it is previously known to utilise various sensor systems by which the optical intensity of a ray of light, conveyed through an optical fibre and coming in towards a sensor element, is influenced due to changes in the respective physical parameter. Such a system may for example be used when measuring the blood pressure in the veins of the human body. Said system is based upon a transformation from pressure to a mechanical movement, which in turn is transformed into an optical intensity, conveyed by an optical fibre, which is in turn transformed into an electrical signal that is related to the measured pressure.
According to known art, such a fibre-optical measurement system may comprise a pressure sensor, an optical fibre connected to said pressure sensor, and at least one light source and at least one light detector located at the opposite end of the fibre, in order to provide the pressure sensor with light, and to detect the information-carrying light signal returning from the pressure sensor, respectively.
One problem occurring with previously known systems of the above kind relates to the fact that interference may occur in the signal transmission path, for example caused by fibre couplings or through bending, intentionally or unintentionally, of the fibre. Already at a light deflection of the fibre, a reduction of the light signal occurs. This signal damping, caused by the bent fibre, entails that the light signal detected in the light detector, which is related to the pressure detected in the sensor element, will have a value that does not coincide with the real pressure. The size of the deviation will then depend on how much the fibre was deflected.
Through EP 0 528 657 A2 a fibre-optical measurement system for measuring pressure is known. Said system comprises a pressure sensor with a membrane, three LED:s emitting light at different wavelengths, and two photo detectors. The system is arranged so that a computing algorithm is used for correction of such temperature effects that may have been superimposed on the output pressure signal. This algorithm is based upon the relationship between membrane deflection, pressure and temperature. Correction data obtained experimentally may also be used as input data to the algorithm regarding temperature compensation.